Three hypotheses for early somatic commitments in Drosophila melanogaster propose 1) a micromosaic of localized determinants; 2) monotonic anterior-posterior and dorsal-ventral gradients; 3) sequential subdivision of the egg into anterior or posterior, middle or end, dorsal or ventral compartments, etc., such that each terminal compartment is specified by a unique binary combinatorial code word reflecting the alternative commitments taken in its formation. To test these, we shall continue chimeric embryo studies. These analyze the adult somatic fate of heterotopically transplanted syncytial blastoderm nuclei, exposed to heterotopic nuclei and cytoplasm destined for different fates. These experiments, with respect to a first mid-egg anterior-posterior compartmental boundary discovered by us, test the compartmental hypothesis that clone restrictions reflect somatic commitments, that presently observed preblastoderm anterior or posterior nuclear commitments are heritable over nuclear division cycles prior to cellularization, that the anterior half syncytial egg is a single "anterior" compartment, that commitments occur successively and combinatorially, or as predicted by the micromosaic or monotonic models. To further test these alternatives we propose to search for predicted patterns of gene activity specific to single imaginal discs or subsets of discs, and to single, or subsets of embryonic segments using cDNA clone banks to polyA+ RNA from pooled discs, the segmented, and blastoderm embryos. Search for candidate regionally specific embryonic sequences will use three independent approaches: 1) disc specific sequences also present in the early embryo; 2) "switch on" sequences in the segmented embryo; 3) mutants lacking specific embryonic segments. Embryonic regionalization of candidates will be tested by in situ and colony hybridization. Discovery of regionally specific polyA+ RNA sequences will allow higher precision tests of the alternative models in chimeric embryo studies of the autonomy of gene expression programmes in heterotopically transplanted nuclei. Expected patterns of non-autonomy afford the start of reliable bioassays to isolate commitment triggers.